Downhole stimulation techniques include high energy-based downhole stimulation techniques and propellant-based downhole stimulation techniques. Such downhole stimulation techniques generally are implemented to increase the effective surface area of producing formation material available for production of hydrocarbons resident in the formation by opening and enlarging cracks in the rock of the formation.
High energy-based downhole stimulation techniques generally employ the detonation of high energy explosive material within a wellbore. The resultant shockwave caused by detonation of the high energy explosive material in the wellbore may be employed to fracture a formation adjacent the wellbore.
Propellant-based downhole stimulation techniques generally employ tools having a circular cylinder housing filled with propellant grain, which may comprise a single volume or a plurality of propellant “sticks” in a housing. Such tools may include conventional propellant ignition systems that use pyrotechnic initiators or small rocket motors. When deployed in a wellbore adjacent a producing formation, the ignition systems will generally initiate a burn at one end of the propellant grain (i.e., a cigarette-type burn) that must propagate along the entire length of the propellant grain. As the propellant grain is initiated, gases from the burning propellant grain exit the housing through holes formed in the housing, entering the producing formation. The pressurized gas may be employed to fracture a formation, to perforate a formation when spatially directed through apertures in the housing against the wellbore wall, or to clean existing fractures or perforations in a formation made by other techniques.
Alternatively, the housing of the tool may include an axially extending bore through the center of the propellant grain and a detonation cord extending through the bore. When deployed in a wellbore adjacent a producing formation, the detonation cord will initiate a burn along the axially extending bore through the center of the propellant grain that will propagate generally radially through the ignited propellant grain. As the propellant grain is initiated, gases from the burning propellant grain exit the housing through preformed holes (which may be initially closed by a thinner housing wall, by a so-called “burst disk,” or by another covering structure to prevent propellant contamination by wellbore fluid) formed in the housing, entering the producing formation. The pressurized gas may be employed to fracture a formation, to perforate a formation when spatially directed through apertures in the housing against the wellbore wall, or to clean existing fractures or perforations in a formation made by other techniques.
U.S. Pat. No. 8,033,333 to Frazier et al., the disclosure of which is incorporated herein in its entirety by this reference, discloses such a propellant-based downhole stimulation device including a detonation cord. The downhole stimulation device includes a housing holding a propellant therein. A detonation cord is disposed in an axially extending bore through the center of the propellant. Initiation of the detonation cord ignites the propellant. Openings or holes in the housing, which are generally initially sealed, serve as passageways for the expelled gas from the ignited propellant to exit the housing into the wellbore.
However, conventional propellant ignition systems that use pyrotechnic initiators or small rocket motors generally provide initiation of the propellant housed therein over a period of ten to one hundred milliseconds. Such a relatively long ignition period as compared to a much shorter period of ignition of high explosive materials may not be desirable in some applications. Such a relatively long ignition period renders impractical and ineffective any contemplated use of the two types of stimulation devices in combination as the ignition of the propellant grain would start well after the detonation of the explosive materials. Thus, formation fractures opened by detonation of an explosive in the wellbore might partially or completely collapse before gas could emanate from a propellant-based stimulation tool to desirably extend and enlarge the fractures. Further, the relatively slower burn of the propellant grain traveling from one end of the propellant grain to the other opposing end also requires relatively more time to build the desired pressures in the housing of the tool. Such a relatively longer time domain to build pressure within the housing may also render impractical and ineffective any contemplated use of propellant-based and explosive-based stimulation devices in combination as the gases produced by combustion of the propellant grain would start exiting the housing well after the detonation of the explosive materials, again negating any potential benefit of deploying propellant-based stimulation tools in the same wellbore as explosive-based stimulation tools. Further, gases produced by combustion of the propellant grain in a conventional propellant-based stimulation tool employing preformed holes in the tool housing to exit gas generated within the tool will exit the tool at a relatively low pressure, reducing the potential benefit of fracture expansion.
Other conventional downhole stimulation devices including initiation systems including a detonation cord, such as those disclosed in U.S. Pat. No. 8,033,333, may not produce the desired pressures in the tool within the desired time domain when implemented in systems including both propellant-based and explosive-based stimulation. Further, the preformed openings in the housing may not allow the desired pressures to build in the housing of the tool as gases will start exiting the housing through the openings once the initial seals are breached. Further still, the initiation of the propellant grain along the bore formed in the propellant grain requires a reduction in overall propellant grain in the housing in order to form the bore, thereby, restricting the amount of propellant that is available in the housing to combust. Finally, the initiation of the propellant grain along the bore formed in the propellant grain may also require relatively more time to build the desired pressures in the housing of the tool as the bore within the propellant grain forms a void in the propellant grain that must be pressurized along with the remainder of the housing.